The invention relates to lasers producing substantially monochromatic radiation of predetermined wavelength of the order of 100 .ANG..
Development of a soft X-ray laser, defined herein as a laser producing substantially monochromatic radiation at a wavelength of the order of 100 .ANG., is an attractive spectre and should be easier to achieve than development of lower wavelength X-ray lasers. Some of the experimental results announced at earlier times have, upon later examination, been found to be inoperable as regards production of X-rays by the originally announced mechanisms. One of the earliest patents in this area is U.S. Patent No. 3,484,721 to Bond and Duguay, wherein a sequence of three or more crystals with plane faces are positioned at the vertices of a three-dimensional polygon, with the puckering or offset angle of each crystal set at the Bragg angle appropriate to the particular X-ray wavelength to be reflected and amplified. The crystal faces are tilted alternately toward and away from the center of the resonator, and one leg of the polygonal path thus defined includes an active medium for amplification of the X-rays reflected (by Bragg diffraction) from the crystal faces comprising the optical cavity. The invention is drawn primarily to an optical cavity suitable for reflecting X-rays around the circuit, and no attention is given to particular gaseous species or energy levels thereof to be used in the amplifier medium.
Another X-ray laser invention is described in U.S. Pat. No. 3,882,312 to Kepros, Eyring and Cagle, wherein near infrared radiation (.lambda.=1.06 .mu.m) is focused by a cylindrical focusing lens on a gel containing a suspension of metal sulfates MSO.sub.4, where M=Cu, Fe or Zn. The laser is not swept across the gel, but is set at an angular orientation .theta.=6.degree.-25.degree. relative to the normal to the gel surface, and the laser delivers 30 Joules of in a 20-nanosecond time interval. Medical X-ray film, positioned at one end of the gel-suspension mixture, and protected against exposure by soft X-rays through use of 1-4 layers of 13-micron thick aluminum foil, was found to be blackened by what was believed to be coherent (laser) X-radiation induced in the sulfates. However, efforts to reproduce this phenomenon by other workers in the field were inconclusive, and Billman and Mark, 12 Applied Optics 2529 (November 1973) offered an alternative explanation that suggests that the X-rays came from internally heated, radiative plasma in the sulfate, rather than from coherent laser X-radiation. Further, Boster, in 12 Applied Optics 433 (1973), has reported results of an experiment carried out at the University of Utah facility, where the Kepros, et al invention was conceived; the results are inconsistent with production of coherent soft X-radiation, but are consistent with electrostatic discharge or turboelectric effects that produce the random local ionization spots on diagnostic photographic film used in the experiment.
Dawson, in U.S. Pat. application Ser. No. 499,221 (filed Aug. 21, 1974), describes another X-ray laser invention, wherein an intense burst of primary X-rays is used to produce a laser-activated plasma by stripping electrons from the high Z atoms in the laser medium, with the low Z atoms in the laser medium retaining their electrons, and hence remaining neutral. The neutral atoms then transfer electrons to highly excited states of the highly stripped, high Z ions, resulting in an inverted population in the high Z ions that produces coherent, monochromatic secondary X-rays as the high energy level orbital electrons return to the ground state. This approach requires the physical transfer of particles (loosely bound electrons) from one atomic species to another within the laser medium, rather than direct optical pumping of one species by an adjacent atomic species. X-ray laser action by this mechanism is yet to be demonstrated.
Mourier, U.S. Pat. No. 3,879,679, discloses a Compton effect laser that utilizes scattering of photons of initial frequency .nu..sub.1 by fast-moving electrons to generate scattered photons of controllable frequency according to the relationship .nu..sub.2 =4.nu..sub.1 (E/E.sub.o) .sup.2 ) where E.sub.o denotes the rest energy of the electron, and E is the relativistic energy of the electron in motion. The Mourier invention requires provision of an electron storage ring or the like for rapidly moving electrons and an optical cavity, associated with the storage ring, for causing photon-electron scattering.
Yariv, in U.S. Pat. No. 3,967,213, teaches the use of a single crystal in the form of a thin film, with a suitably oriented set of atomic planes as an X-ray laser. When the crystal is pumped, X-ray photons emitted from one of the atomic constituents of the crystal experience Bragg scattering from the atomic planes, and the system thereby provides its own internal feedback for coherent pumping within the laser medium. The pump radiation enters the crystal substantially parallel to the aforementioned atomic planes, and the incident radiation to be amplified moves substantially transversely to the atomic planes from one side of the system to the other. The Yariv invention utilizes (solid) crystals of the Zincblende class, such as GaP, or more generally Ga.sub.1-x A.sub.x P, where A is a third atomic constituent of the crystal.
Dixon and Elton, in 38 Phys. Rev. Letters 1072 (1977), have reported on a method for production of very soft X-radiation (.lambda..gtoreq.800 .ANG.), using resonance charge transfer and resulting population inversion of the product ion excited state(s), using carbon atom-carbon ion interactions in a laser-generated plasma. One such reaction is C +C.sup.5+,6+ .fwdarw.C.sup.+ +(C.sup.4+,5+).sup.*, and (C.sup.4+,5+).sup.* .fwdarw.C.sup.4+,5+ +h.nu., with E =h.nu.=10-20 eV. This requires relative reactant velocities of v=10.sup.6 -10.sup.8 cm/sec, and it is unclear how monochromatic is the emitted radiation, since radiative cascade from the resulting excited state to all lower states, including ground, is observed.
Shatus et al, in U.S. Pat. No. 3,746,860 disclose a soft X-ray generator, using a high energy pulsed laser, a high Z material of modest optical depth and a high density plasma, these three elements being axially aligned in that order. The laser radiation passes into and is absorbed by the high Z material, which produces high Z ions that pass into the plasma for enhancement of plasma x-radiation. The X-radiation emitted from the plasma is not even approximately monochromatic, but varies over all X-ray wavelengths consistent with electron capture and other electric transitions involving the injected high Z ions.
U.S. Pat. No. 3,813,555 to Viecelli teaches the use of a highly relativistic ion beam (V.sub.ion =(1-.epsilon.)c with .epsilon.&lt;&lt;1) interacting with a nonaligned visible wavelength laser to produce nearly monochromatic X-radiation in the forward direction by Doppler shift, time dilation and length compression. The laser pump (frequency .nu..sub.p) is aligned at an angle .theta. given by ##EQU1## This apparatus requires use of a relativistic ion beam and careful control of the pump laser angle of incidence.
An X-ray laser utilizing flash heating of an intermediate Z material to K-shell binding energy temperatures is disclosed in U.S. Pat. No. 3,823,325 to Wood. A laser pulse of duration .DELTA.t.ltorsim.10.sup.-12 sec and energy E.apprxeq.1 Joule produces flash heating and subsequent radial expansion in a filament of diameter .ltorsim.1 .mu.m, with the laser pulse being arranged to travel along the wire at the group velocity of light and thus produce a moving region of population inversion in the filament material. The electron decay produces coherent radiative emission, moving in the direction of and thus reinforced by the moving pump pulse of decay energy E.apprxeq.100 eV (.lambda..apprxeq.125 .ANG.) with associated decay times of 10.sup.-11 sec. The initial (pump) pulse must be of narrow wavelength and optically shaped and cannot utilize generally broadband radiation that is used in the subject invention.
McCorkle and Joyce, in U.S. Pat. No. 4,042,827, teach the use of a heavy ion, 1 keV pump beam, swept along the length of a thin metal target at approximately the group velocity of light in the target material so as to selectively create atomic inner shell vacancies, with the shell vacancy region forming a moving wavefront. The substantially monochromatic radiation emitted by decay of the excited atoms (e.g., through Auger filling of the inner shell vacancy) moves with the shell vacancy moving wavefront and gives rise to a single pass, cavity-less X-ray laser of wavelength of the order of 50 .ANG., dependent upon the relationship of the target material K-edge, L-edge, etc. to the ion energy, with a quantum efficiency of the order of 5%. The ion energy used as a pump must be of narrow wavelength and cannot utilize broadband radiation that is used in the subject invention.
An X-ray laser utilizing a gas jet or a thin foil for the target material is disclosed in U.S. Pat. No. 4,053,783 to Scully. With the gas jet approach, a jet of hydrogen gas flows longitudinally between and parallel to two strip transmission lines that are connected to a high voltage (10 keV) pulse source. An ion beam generator injects a particle beam such as He.sup.++ or Li.sup.++ at an energy of 300 eV between the strip transmission lines, and the high voltage pulser is activated. An E field wavefront then moves along the length of the strips at the speed of light, causing the ion beam to deflect and impinge on the gas stream at a predetermined position behind the moving E field wavefront. As the beam strikes the gas jet particles, resonant charge exchange produces a population inversion (2p-1s) in the ions, with the gas jet density (n.apprxeq.5.times.10.sup.16 /cm.sup.3) adjusted to maximize one electron pickup in the 2p state in one atom decay length. The Scully invention requires use of an ion beam within a narrow band of initial kinetic energies and use of a high voltage pulse source, inter alia.
Vinogradov, Skobelev and Yukov, in Sov. Jour. of Quantum Electronics, 6 525(1975), noted that He-like ions of charges Z=5-11 may be pumped by ultraviolet radiation to produce a population inversion between n=3 and n=2 levels or between n=4 and n=3 levels with characteristic inter-level decay times of the order of a few picoseconds. In contemporaneous publications, such as Sov. Jour. of Quantum Electronics, 5 630 (1975), Vinogradov et al suggest the use of resonant line pairs for pumping short wavelength lasers, using a Si-Al resonance as an example; but they do not indicate or even speculate on how one would provide the resonance line radiation for laser pumping, given the constraints such as picosecond decay times and absorption of the pump radiation by intervening media.
Norton and Peacock, in Jour. of Phys. B, 8 989 (1975), suggest the use of opacity-broadened lines to compensate for lack of precise line pair coincidence in resonance line radiation pumping, using the highly ionized species C.sup.5+ as an example. Bhagavatula, in Jour. of Appl. Phys., 47 4535 (1976) has proposed the use of resonant line radiation from identical or different ion species to populate an upper laser level in a species such as Mg XII (pumped by C VI ions) to produce laser gains .apprxeq.100 cm.sup.-1 on the 4-3 transition at .lambda..apprxeq.130 .ANG.; hydrogen-like, helium-like and lithium-like ions are examined for resonance pairs with small resonance mismatches. Again, no means of delivering the resonance line radiation to the laser medium, within the physical constraints imposed at short wavelength operation, is proposed.